Carbon nanotube devices with unzipped low-resistance contacts

ABSTRACT

A method of creating a semiconductor device is disclosed. An end of a carbon nanotube is unzipped to provide a substantially flat surface. A contact of the semiconductor device is formed. The substantially flat surface of the carbon nanotube is coupled to the contact to create the semiconductor device. An energy gap in the unzipped end of the carbon nanotube may be less than an energy gap in a region of the carbon nanotube outside of the unzipped end region.

BACKGROUND

The present invention relates to carbon nanotube field effecttransistors (FETs), and more specifically, to improving electricalcoupling between contact metals and carbon nanotubes in carbon nanotubefield effect transistors.

Carbon nanotube field-effect transistors generally include a carbonnanotube that spans a gap between a source contact and a drain contactand serves as the channel of the transistor, the conductance of which ismodulated by a gate separated from the nanotube channel by a dielectricmaterial. There exists a contact resistance at the interfaces betweenthe carbon nanotube and the source/drain contacts due, in part, todifficulties in coupling the cylindrical surface of the carbon nanotubeto the contacts. High contact resistance at the interface between thecarbon nanotube and either of the contacts will reduce current injectioninto the nanotube channel, thereby decreasing the performance of thetransistor. This problem of contact resistance is exacerbated attechnologically relevant nanotube diameters that are less than about 2nanometers (nm) where a Schottky barrier presents itself at the contactmetal/nanotube interface. The band gap of the carbon nanotube increasesinversely with respect to diameter and a sufficiently large bandgap(˜0.6 eV) is necessary to attain a suitable on-/off-current ratio fordigital applications. However, decreasing the diameter to achieve thisbandgap leads to larger Schottky barriers and weaker coupling betweenthe contact metal and the carbon nanotube thus increasing the contactresistance.

SUMMARY

According to one embodiment of the present invention, a method creatinga semiconductor device includes: unzipping an end of a carbon nanotubeto provide a substantially flat surface; forming a contact of thesemiconductor device; and coupling the planar surface of the carbonnanotube to the contact to create the semiconductor device.

According to another embodiment of the present invention, a method ofcreating a carbon nanotube field-effect transistor includes: altering aphysical structure of a segment of the carbon nanotube to reduce anenergy gap in the altered segment; and coupling the altered segment to acontact to create the carbon nanotube field-effect transistor.

According to another embodiment of the present invention, a method ofreducing a contact resistance in a carbon nanotube transistor includes:unzipping a segment of a carbon nanotube; forming a planar surface fromthe unzipped segment of the carbon nanotube; and coupling the planarsurface to a contact of the carbon nanotube transistor.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIGS. 1-6 illustrate an exemplary manufacturing process for creating anexemplary carbon nanotube field effect transistor in one embodiment ofthe present invention, in which:

FIG. 1 illustrates a starting substrate structure including a co-planarlocal bottom gate formed therein, a dielectric layer formed over thebottom gate, and a carbon nanotube deposited on the dielectric layer andaligned to the local bottom gate;

FIG. 2 illustrates a barrier material and a photoresist layer formedover the structure of FIG. 1;

FIG. 3 illustrates windows formed in the barrier material and aphotoresist layer of FIG. 2 that expose ends of the carbon nanotube.

FIG. 4 illustrates unzipped ends of the carbon nanotube, which areformed by chemical treatment of the nanotube in the exposed window areasof FIG. 3;

FIG. 5 illustrates resist layer having windows for formation ofsource/drain contacts on the unzipped ends of FIG. 4;

FIG. 6 illustrates contact metal deposited onto the unrolled ends of thecarbon nanotube;

FIG. 7 shows an exemplary contact formed using the exemplarymanufacturing processes disclosed herein; and

FIG. 8 shows an exemplary band gap diagram of a carbon nanotube that hastwo ends that have been unzipped and to lie flat as disclosed herein.

DETAILED DESCRIPTION

FIGS. 1-6 illustrate an exemplary manufacturing process for creating anexemplary carbon nanotube field effect transistor in one embodiment ofthe present invention. FIG. 1 illustrates a first stage of themanufacturing process. A substrate 102 is provided that has a co-planarlocal bottom gate 104 formed therein, filled with a bottom gatematerial, such as tungsten, for example. The substrate 102 may includesilicon dioxide (SiO₂) in various embodiments. Top surfaces of thesubstrate 102 and of the local bottom gate 104 are substantially alignedto form a substantially planar surface using, for example, chemicalmechanical polishing. A high-k dielectric layer 106 is formed on theplanar surface of the substrate 102 and local bottom gate 104. Thehigh-k dielectric layer 106 may be deposited using, for example, atomiclayer deposition or other suitable deposition methods. Exemplary high-kdielectric material may include HfO₂ and Al₂O₃, among others. The high-kdielectric layer 106 may provide electrical isolation of the underlyinggate 104 from transistor source and drain metals formed in latermanufacturing processes. A carbon nanotube 108 is deposited on a toplayer of the high-k dielectric material 106. The carbon nanotube 108 isgenerally aligned with the local bottom gate 102 so as to span the localbottom gate 104, such that the ends of the carbon nanotube 108 extendbeyond the local bottom gate 104 and over the silicon substrate 102.

FIG. 2 illustrates a second stage of the manufacturing process. An etchbarrier material 110 is deposited over the top surfaces of the high-kdielectric layers 106 and the carbon nanotube 108. In variousembodiments, the barrier material 110 may be Si₃N₄, Al₂O₃ or othersuitable barrier material. The etch barrier material serves as both aprotective layer in later manufacturing steps to prevent chemicalunzipping of the nanotube in a channel region of the carbon nanotube(discussed below) and a means to self-align the contact metals to theunzipped portion of the nanotube in the contact areas of the carbonnanotube (discussed below). A resist layer 112 is then formed on top ofthe barrier material 110.

FIG. 3 illustrates a third stage of the manufacturing process. In thisstage, windows 114 a and 114 b are formed in the resist layer 112 andtransferred to the barrier material 110 by means of chemical etching.Windows 114 a and 114 b may be formed by standard lithographicprocessing of the resist layer 112 followed by wet etching of thebarrier material 108 a. In this way, the pattern defined in the resistlayer 112 by windows 114 a and 114 b is transferred through exposedportions of the barrier material 110 so as to expose ends 108 a and 108b of the carbon nanotube 108, while also still covering a middle segmentof the carbon nanotube 108 that forms the channel region 108 c of thecarbon nanotube 108.

FIG. 4 illustrates a fourth stage of the manufacturing process. Theresist layer 112 is removed and the ends of the carbon nanotube 108 inthe exposed window areas 114 a and 114 b are unzipped and unrolled toform a planar surface of contact regions 108 a and 108 b, as disclosedherein. The planar surface is then disposed to lie substantially flatagainst the high-k dielectric layer 106. In one embodiment, unzipping anend of the carbon nanotube 108 includes forming a cut in the end of thecarbon nanotube. In an exemplary embodiment, the cut is along alongitudinal direction of the carbon nanotube 108. Alternately, the cutmay be along a spiral direction of the carbon nanotube 108. The cut maybe formed by subjecting the exposed carbon nanotube contact regions 108a and 108 b of the carbon nanotube 108 to a potassium permanganate(KMnO₄) mixture, for example. The potassium permanganate breaks bonds inthe surface of the carbon nanotube 108, thereby creating two free edgesof the carbon nanotube 108 in the contact regions 108 a and 108 b. Thefree edges include dangling bonds that enable the establishment ofcovalent bonds between contact metal (shown in FIG. 6) and the nanotubei.e. edge contacted devices. The barrier layer 110 may include a wettinglayer to reduce air gaps between the barrier layer 110 and the carbonnanotube 108 to prevent access of the potassium permanganate or otherchemical treatment to the carbon nanotube channel region 108 c. As shownbelow with respect to FIG. 8, preserving the channel region 108 c in itscylindrical form preserves the bandgap in this region.

FIG. 5 illustrates a fifth stage of the manufacturing process. Resist116 is applied to the surface of the transistor and developed to reopenwindows 114 a and 114 b in the contact regions of the carbon nanotube108, i.e., at the unrolled ends 108 a and 108 b of the carbon nanotube108.

FIG. 6 illustrates a sixth stage of the manufacturing process. Contactmetal 120 is deposited in the window areas 114 a and 114 b onto thecontact regions 108 a and 108 b of the carbon nanotube 108. The contactmetal 120 such as palladium for p-type or erbium for n-type transistorsmay be deposited by electron-beam deposition or other suitable method.The contact metal 120 thus couples to planar surfaces of the contactregions 108 a and 108 b. Additionally, the contact metal may couple tothe free edges of the contact regions 108 a and 108 b, thereby formingcovalent bonds with the dangling bonds of the free edges. The resist 116is then lifted off of exposed surfaces to complete the carbon nanotubefield effect transistor. A subsequent anneal at temperatures in therange from about 200° C. to about 400° C. in one of N₂, forming gas, orvacuum may provide further improvements in contact resistance.

FIG. 7 shows an exemplary contact 700 formed using the exemplary processdisclosed herein. Carbon nanotube 702 includes a cylindrical channelregion 702 a and an unzipped contact region 702 b. The cylindricalchannel region 702 a spans the gated section of the transistor. Theunzipped contact region 702 b is laid substantially flat against thehigh-k dielectric 704 and contact 706 is formed thereon. The width W ofthe unzipped contact region 702 b is related to the diameter d_(CNT) ofthe carbon nanotube (W=πd_(CNT))_(.) The unzipped contact region 702 band the contact 706 form a substantially seamless contact. Currententers the unzipped contact region 702 b from the contact 706 either atthe free edges 710 (as shown by exemplary current 720) or through thesurface 712 (as shown by exemplary current 722).

FIG. 8 shows an exemplary band gap diagram 800 of a carbon nanotube thathas ends that have been unzipped and flattened as disclosed herein toform a carbon nanotube field-effect transistor. The exemplary band gapdiagram 800 includes a first bandgap region 802 that corresponds to thebandgap of the cylindrical channel region 702 a of the carbon nanotubeof FIG. 7, also referred to herein as the channel region. The exemplaryband gap diagram 800 also includes second bandgap regions 804 thatcorrespond to a band gap diagram for the unzipped contact regions 702 bof the carbon nanotube of FIG. 7. In the band gap diagram 800, E₀ is avacuum energy level. Energy gap E_(G1) is an energy gap of the carbonnanotube in the cylindrical channel region 702 a and is the energydifference between a valence band energy level E_(V1) and the conductionband energy level E_(C1) in the channel region. In various aspects, theenergy gap E_(G1) is inversely proportion to a diameter of the carbonnanotube (E_(G1)˜1/d_(CNT)). E_(G2) is the energy gap in the unzippedregion of the carbon nanotube that forms the contacts to the sourceand/or drain. The E_(G2) is the energy difference between valence bandenergy level E_(V2) and conduction band energy level E_(C2).

In an exemplary embodiment, energy band gap E_(G2) in unzipped region702 b is less than the energy band gap E_(G1) in the cylindrical channelregion 702 a. For example, for a 1.2 nm carbon nanotube, the band gapvalues for the carbon nanotube channel region E_(G1) ranges from about650 meV to about 700 meV and the width of the unzipped region isapproximately 3.8 nm. Experimentally, the band gap of the unzippedcontact region E_(G2) is Eg=α/W, wherein α=0.7-1.5 i.e. the bandgap of agraphene nanoribbon. Thus, the band gap values for the unzipped regionE_(G2) ranges from about 185 meV to about 395 meV, roughly half of thechannel bandgap E_(G1).

E_(FM) is the Fermi level of the metal or a potential energy level foran electron as defined by the Fermi-Dirac distribution, for example.φ_(m) is a metal work function of the contact metal or the energyrequired to move an electron from a Fermi energy level to the vacuumlevel. A Schottky barrier is denoted by φ_(B). The Schottky barrierφ_(B) is defined as the difference between a minimum in the conductionband and the metal Fermi level E_(FM) for an n-type semiconductor and isa difference between the valence band maximum and the metal Fermi levelfor a p-type semiconductor. The Schottky barrier arises from a mismatchbetween the semiconductor Fermi level and the contact metal Fermi levelat the interface. The Schottky barrier φ_(B) generally is present whenthe band gap is within a range that is useful for digital transistors˜0.6 eV. The Schottky barrier generally inhibits current injection intoa valance band for a p-type device. As shown in the exemplary band gapdiagram 800, the Schottky barrier rises to a level above E_(V2). Thusfor a carbon nanotube device without unzipped or flattened ends (i.e.,where E_(V2) =E_(V1) and E_(G2)=E_(G1)), the Schottky barrier inhibitsinjection of electrons directly into the contact region 702 b. Similararguments can be made for inhibiting injection into the conduction bandfor an n-type device. The band diagram 800 also shows that for thesecond bandgap region 804 (where E_(G2)<E_(G1) and E_(V2) is close toE_(FM)) the Schottky barrier is much smaller or vanishes altogether inthe unzipped contact region 702 b. Therefore, reducing the energy gapE_(g) in the second bandgap region 804 lowers the Schottky barrier. As aresult, the contact resistance associated with the Schottky barrier isreduced or eliminated. Thus, an applied bias in the gated region 702 bsimply modulates E_(G1) and therefore the current through the channel asin a conventional metal-oxide-semiconductor (MOS) device.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the exemplary embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A method of forming a semiconductor device, the method comprising:unzipping an end of a carbon nanotube to provide a substantially flatsurface; forming a contact of the semiconductor device; and coupling thesubstantially flat surface of the carbon nanotube to the contact tocreate the semiconductor device.
 2. The method of claim 1, whereinunzipping the end of the carbon nanotube further comprises one offorming a longitudinal cut at the end of the carbon nanotube and forminga spiral cut at the end of the carbon nanotube.
 3. The method of claim1, further comprising coupling a free edge of the substantially flatsurface of the carbon nanotube to the contact.
 4. The method of claim 1,further comprising sandwiching the substantially flat surface of thecarbon nanotube between the contact and a substrate of the semiconductordevice.
 5. The method of claim 4, further comprising disposing thecarbon nanotube on a surface of a high-k dielectric, unzipping the endof the carbon nanotube to provide the substantially flat surface on thesurface of the high-k dielectric, and forming the contact on top of thesubstantially flat surface of the carbon nanotube.
 6. The method ofclaim 1, wherein coupling the substantially flat surface to the contactreduces a contact resistance between the contact and the carbonnanotube.
 7. The method of claim 1, wherein the contact is one of asource contact and a drain contact and the carbon nanotube is a gatedchannel of the semiconductor device.
 8. A method of forming a carbonnanotube field-effect transistor, comprising: altering a physicalstructure of a segment of the carbon nanotube to reduce an energy gap inthe altered segment; and coupling the altered segment to a contact tocreate the carbon nanotube field-effect transistor.
 9. The method ofclaim 8, wherein altering the physical structure of the segment of thecarbon nanotube further comprises unzipping an end segment of the carbonnanotube and unrolling the unzipped end segment to form a substantiallyplanar surface.
 10. The method of claim 9, wherein unzipping the endsegment further comprises one of forming a longitudinal cut at the endsegment of the carbon nanotube and forming a spiral cut at the endsegment of the carbon nanotube.
 11. The method of claim 8, furthercomprising coupling an edge of the substantially planar surface to thecontact.
 12. The method of claim 8, further comprising sandwiching thealtered segment of the carbon nanotube between the contact and asubstrate of the field effect transistor.
 13. The method of claim 8,wherein coupling the altered segment to the contact reduces a contactresistance between the contact and the carbon nanotube.
 14. The methodof claim 8, wherein the contact is one of a source contact and a draincontact and the carbon nanotube is a gated channel.
 15. A method ofreducing a contact resistance in a carbon nanotube transistor,comprising: unzipping a segment of a carbon nanotube; forming a planarsurface from the unzipped segment of the carbon nanotube; and couplingthe planar surface to a contact of the carbon nanotube transistor. 16.The method of claim 15, wherein unzipping the segment of the carbonnanotube further comprises one of forming a longitudinal cut at an endof the carbon nanotube and forming a spiral cut at an end of the carbonnanotube.
 17. The method of claim 15, wherein coupling the planarsurface to the contact further comprises forming a covalent bond betweenthe contact and a dangling carbon bond at an edge of the planar surface.18. The method of claim 15, further comprising sandwiching the planarsurface between a substrate and the contact.
 19. The method of claim 15,wherein altering the physical structure of the segment reduces an energygap in the unzipped segment with respect the energy gap of the carbonnanotube outside of the unzipped segment.
 20. The method of claim 15,wherein the contact is one of a source contact and a drain contact andthe carbon nanotube is a gated channel. 21-40. (canceled)